1. Technical Field
The present invention relates generally to audio systems, and more specifically a closed loop controller for an audio speaker.
2. Description of the Related Art
Numerous improvements have been made over the years in audio reproduction systems to improve performance and audio quality. Amplifier and loudspeaker designs have improved dramatically to provide better response and lower distortion of the audio signal. Significant research continues in these and other areas to improve overall audio system performance.
In recent years, digital storage of audio programs has become increasingly popular. Digital audio disks (usually referred to as CDs) have become well established in the marketplace. Digital audio tape (DAT) is gaining increasing marketplace acceptance. Digital audio storage has a number of advantages over traditional analog storage methods. With the use of error correcting codes, digital storage of audio programs and their subsequent retrieval is substantially distortion free. In addition, some media, such as CDs, do not suffer wear with use as do traditional analog media such as LP records.
However, the use of digital storage media does not solve the problem of signal distortion during playback. The digital signal must be converted to an analog signal for amplification. Additionally, signal conditioning techniques such as frequency band equalization are often performed on the converted analog signal. As is well known in the art, various types of distortion of the original signal are introduced by these components.
It is also well known in the art that speaker systems are generally the portion of the overall system which is the most difficult to manufacture so as to provide distortion-free signal reproduction. This is because loudspeaker systems are electro-mechanical systems, and the mechanical portion of the system has numerous modes which can introduce distortion into the reproduced audio signal. These include flexure of various loudspeaker parts, and mechanical resonances which cause the reproductive efficiency of the speaker to vary with frequency. Expensive speaker systems can be built which help minimize these and other distortions, but the complexity and cost of such systems prohibits their widespread use.
Various prior art systems have been designed in an attempt to compensate for speaker and other distortion added to the audio signal. For example, attempts have been made to monitor the reproduced audio signal at the speaker or in the listening area, with the gain of the amplifier at various frequencies being changed dynamically. Examples of this approach can be found in U.S. Pat. No. 4,327,250, DYNAMIC SPEAKER EQUALIZER, issued to von Recklinghausen, and U.S. Pat. No. 4,610,024, AUDIO APPARATUS, issued to Schulhof.
Another approach is to carefully determine the characteristics of each speaker after manufacture, and store this information in a read only memory. Using this data, a signal can be added to the audio signal in a microcomputer to pre-distort the audio signal. This predistortion theoretically cancels the effects of the speaker. An example of such an approach is shown in U.S. Pat. No. 4,852,176, CONTINUOUS DIFFERENTIAL SIGNAL EQUALIZER, issued to Truhe, Jr.
One drawback of approaches such as those described above is that they are relatively complex and expensive. Although use of such techniques can improve the performance of the audio system, there remains room for improvement.
It would therefore be desirable to provide a controller for an audio speaker which provides a more accurate reproduction of an original audio signal. It would further be desirable for such a controller to utilize digital input signals directly, so that distortion caused by analog components is eliminated.